1. Field of the Invention
The present design relates generally to the art of field-sequential stereoscopic liquid crystal display, and more specifically to shuttering eyewear used in conjunction with liquid crystal displays that enable enhanced image brightness and resolution, as well as ease of viewing.
2. Description of the Related Art
Current stereoscopic display systems most often employ individual eyewear selection devices for proper image viewing. Selection devices employed with these systems are either active (shuttering) or passive (polarizing) eyewear. People working in molecular modeling and in certain CAD presentation applications tend to prefer using active eyewear selection devices. People working with geographic information systems, including measurement and aerial mapping applications, tend to favor a passive eyewear solution.
Active selection devices provide shuttering functionality within the eyewear. When an observer looks through shuttering eyewear, his or her entire visual field is subjected to a duty cycle, or to shuttering. When an observer looks through passive eyewear, no shuttering occurs in the non-monitor visual field, but instead the combination of the eyewear and the monitor's electro-optical modulator creates the shuttering functionality of the display. Because the non-monitor visual field is not subjected to a shuttering duty cycle, a lower field rate may be used since there is no opportunity for brightly lit environments to flicker under such circumstances.
Field rate is related to the frame rate, where the field rate for conventional television displays for an interlaced video image is two times the frame rate. Interlacing provides one half of the image to the display at a time, normally broken into odd and even numbered lines. A field rate of 90 Hz corresponds to a 45 frame per second video image. While interlaced signals may have more than two fields per frame, no such signal is commonly employed.
One design of a current stereographic display system that exhibits the lower field rate advantage is realized using a passive eyewear selection device in conjunction with a liquid crystal modulator such as StereoGraphics Corporation's ZScreen®. The ZScreen is a liquid crystal modulator described by Lipton et al. in U.S. Pat. No. 4,792,850, and first manufactured by StereoGraphics Corporation. In this design, the modulator serves to change the characteristics of polarized light at the video field rate such that a sequence of fields having alternatively left- and right-handed circularly polarized light are emitted by the modulator/monitor combination. The left and right images are encoded with a polarization characteristic that may then be decoded by the passive eyewear consisting of a circularly polarizing analyzer for the left and right eyes respectively. The passive eyewear described in the above configuration may also be used in conjunction with planar monitors or other stereoscopic monitors that are not necessarily “genlocked,” or field rate synchronized, to, or in phase with, the stereoscopic monitor. Genlocking is employed since, unlike active eyewear, no shuttering occurs when a user looks at planar monitors, and these monitors cannot exhibit unwanted and disturbing flicker.
The major reason a lower field rate implementation is advantageous is because of the relationship between field rate and effective resolution. The lower the field rate, the higher the effective resolution of the stereoscopic display system. This relationship is a result of a display/monitor device, including Cathode Ray Tubes (CRTs) in particular, having a fixed bandwidth that limits the number of pixels per unit of time can be written. Stereoscopic display systems operating at a higher field rate tend to limit or lower the number of pixels that can be written for a field or frame, and consequently yield a decrease in resolution. Thus, it may be advantageous in certain applications to operate at a lower field rate to produce a higher resolution image provided that perceived flicker is not increased.
In practice, using a CRT monitor with passive eyewear, a field rate as low as approximately 90 fields per second may produce a satisfactory viewing experience with minimal or zero flicker. In comparison, a satisfactory viewing experience using active or shuttering eyewear requires a significantly higher rate of 120 fields per second. The increased field rate associated with use of shuttering eyewear is necessary to prevent the room environment surrounding the monitor from exhibiting flicker. If shuttering eyewear is used to view stereoscopic images in a bright room at anything less than approximately 120 Hz, room environment flicker will occur. These field rate ranges apply to liquid crystal displays as well.
Shuttering eyewear may become problematic when used in a viewing area containing displays or monitors that are not genlocked to the stereoscopic monitor. Such shuttering eyewear employed in these conditions may produce roll bars or other disturbing visual effects. Such visual effects result where these shutter mechanisms are not synchronized to the planar video sources present.
Passive eyewear solutions are typically employed with an onscreen polarization modulator or switch, and the only shuttering occurs on the display screen. Passive eyewear solutions do not produce shuttering effects when the user views the rest of the visual field or room environment. Therefore, use of passive eyewear in room environments may have a significant advantage over active eyewear designs.
In contrast, passive eyewear designs may become problematic as image viewing screen size increases above 20 inches. Onscreen modulators that are typically constructed with pi-cells or surface-mode devices become costly in large sizes. Onscreen modulators having dimensions larger than 12×16 inches may be employed (this size is suitable for a 19- to 21-inch monitor). If modulators or monitors of a larger size are desired, i.e. larger than 20 inches, the cost of the selection device increases dramatically.
It would therefore be beneficial to provide a stereoscopic display system for use in conjunction with viewing stereoscopic images that overcome the foregoing drawbacks present in previously known active eyewear designs and retains the good characteristics of onscreen modulation. Further, it would be beneficial to provide an active eyewear design that exhibits a lower field rate and higher effective resolution than found in passive eyewear solutions. It would also be advantageous to provide active eyewear having an appreciable cost savings over previously known eyewear designs requiring expensive modulators constructed of pi-cells.